1. Field of Invention
Measurement of cardiac output is crucial in the care of critically ill patients such as patients with multiple trauma, patients in overwhelming sepsis, and patients with acute myocardial infarction. In the case of patients with acute myocardial infarction, there is a worsening prognosis with decrease in cardiac output. Knowledge of the cardiac output provides information useful in determining the clinical state of a given patient and in rationally planning therapy for the patient. Such information is not contained in the usually measured vital signs. For example, a low mean arterial pressure with elevated pulse does not adequately distinguish between cardiogenic and septic shock, the treatments for which are quite different. Consequently, a method that distinguishes between cardiogenic and septic shock would be important in planning appropriate therapy. The measurement of cardiac output, in this case, would provide valuable information that would allow an appropriate diagnosis to be made.
2. Prior Art
The importance of knowing cardiac output has led to many methods for its determination. The most commonly used method in widespread clinical use is thermodilution. In the thermodilution method a catheter is placed into the central venous circulation, usually by percutaneous entry into the internal jugular or subclavian vein. A balloon at the end of the catheter is inflated, and the normal flow of blood is employed to direct the tip of the catheter into the pulmonary artery. Measurement of cardiac output is made by observing the dissipation of a temperature pulse, usually a bolus of iced sterile water or saline solution. As is evident, the method cannot be used without invasion of the vascular tree. Indeed, the catheter is threaded through the heart and the heart valves. Flow direction is not entirely reliable. In certain patients access to the pulmonary artery is impossible. During placement of the catheter cardiac arrhythmias are not uncommon. Other complications include sepsis, thrombosis of the central veins, emboli, and fatal rupture of the pulmonary artery. Other disadvantages of the technique include lack of continuous information about the cardiac output and chance location of the catheter, such as in an unfavorable pulmonary artery branch, with erroneous values for the cardiac output. Analysis of the error inherent in the measurement of blood flow by thermodilution has revealed a standard deviation of 20-30%.
Measurement of cardiac output has also been done by the indocyanine green dye technique, which suffers from several disadvantages. The technique is cumbersome, it requires the placement of an arterial catheter, is not accurate at low levels of cardiac output and is difficult to use for repeated measurements in the same patient. Complications include catheter site hematoma, sepsis from the catheter, thromboses of the artery containing the indwelling catheter, and pseudoaneurysm formation at the site of arterial puncture.
The Fick method is based on the measurement of oxygen consumption. It is best used in awake, alert, stable patients not requiring respiratory support on a ventilator. The method requires invasion of the pulmonary artery in order to obtain samples of mixed venous blood for determination of the oxygen content. Like the indocyanine green dye technique, an arterial catheter must be placed for sampling of arterial blood for oxygen content with the disadvantages mentioned above.
Transcutaneous ultrasound has also been used. Ultrasound transducers are placed externally on the body at the suprasternal notch. Under the most sanguine circumstances, at least 10% of patients cannot have their cardiac outputs measured in this way. Many difficulties with this approach have been reported: repeated measurements may lead to varying location of the sample volume that is scanned, there are changes in the angle of intersection of the ultrasound beam with the axis of the vessel, capability for continuous measurement of the cardiac output is not available, and other major thoracic vessels may interfere with the Doppler ultrasound signals. Further, the method is not feasible in many important clinical settings in which the patients are not cooperative or are in the operating room, where the suprasternal notch may not be accessible.
Because of these difficulties, an implantable, removable Doppler ultrasound device for measurement of the cardiac output has been developed for direct attachment to the aorta. The device requires a major, operative, invasive intervention, such as splitting the sternum or removal of a rib to enter the chest cavity, for placement of the device directly on the wall of the aorta. Removal of the device also requires surgical intervention. If the device were to be lost in a major body cavity, a major surgical procedure would be required.
Measurement of cardiac output by continuous or single breath, gas-washout has been attempted, but is not used in standard clinical medicine. Such methods require many approximations of lung function in modeling the system. Time consuming numerical analysis is required. In one study, measurement of cardiac output in anesthetized patients using argon and freon during passive rebreathing was shown to provide lower cardiac outputs than a simultaneously performed Fick determination. The authors concluded that the method caused significant disturbances of hemodynamics and was therefore not suitable for widespread use.
Indirect measurements include the pulse, blood pressure, and urine output, but these measurements are not specific for cardiac output. For example, in the presence of acute renal failure, urine output cannot be correlated with perfusion of major organs.
In the patent art, Tickner, U.S. Pat. No. 4,316,391 discloses an ultrasound technique for measuring blood flow rate. Colley et al., U.S. Pat. No. 4,354,501, discloses an ultrasound technique for detecting air emboli in blood vessels. Numerous patents disclose catheters or probes, including Calinog, U.S. Pat. No. 3,734,094, Wall, U.S. Pat. No. 3,951,136, Mylrea et al., U.S. Pat. No. Re. 31,377, Perlin, U.S. Pat. Nos. 4,304,239; 4,304,240 and 4,349,031, Colley et al., U.S. Pat. No. 4,354,501 and Furler, U.S. Pat. No. 4,369,794.
U.S. Pat. No. 4,331,156 discloses an esophageal cardiac pulse probe which utilizes a closed end lumen with a pressure transmitting fluid therein to transmit sounds from the heart and lungs to an external transducer.
In U.S. Pat. No. 4,671,295 and 4,722,347 there is described a method and apparatus for measuring cardiac output which comprises placing an ultrasound transducer in great proximity to the ascending aorta of the heart of the mammal by passing a probe carrying the transducer into the trachea and transmitting ultrasound waves from the transducer toward the path of flow of blood in the ascending aorta. The probe can be passed through the nasal or oral cavity, past the epiglottis into the trachea or, in the case of patients who have had a tracheostomy, directly into the trachea through the surgical opening. The reflected ultrasound waves are received by the transducer and the average Doppler frequency difference between the transmitted waves and the reflected waves is measured. The cross-sectional size or area of the ascending aorta at the point of ultrasound reflection is calculated and the volumetric blood flow rate is determined from such measurements. The method and apparatus for measuring cardiac output described in U.S. Pats. No. 4,671,295 and 4,722,347 provides for the determination of the cardiac output in a way that is accurate, noninvasive, continuous, inexpensive and suitable for use in those patients whose cardiac output measurement is most critical.